For dental fillings, use is increasingly being made of dental glass/polymer composites in order to avoid possible side-effects of amalgam fillings and in order to achieve a better aesthetic impression. Dental glass/polymer composites generally consist of an inorganic component and an organic polymer binder. The inorganic component consists predominantly of glass powder. Besides the powder properties necessary for good filling, the glass powder used also has to meet certain requirements regarding the physical and chemical properties of the glass to be used for the powder.
The refractive index of the glass powder must be matched as closely as possible to that of the synthetic resin matrix used in order to imitate the partially transparent appearance of natural tooth enamel and thus to meet the high aesthetic requirements.
Refractive index differences between glass and synthetic resin of greater than 0.05 result in undesirably high opacity of the dental glass/polymer composite and should therefore be avoided. In addition, the glass powder must have good processing properties and favourable setting behaviour during preparation of the composites and must ensure high strength after curing.
It is furthermore important that the thermal expansion of the dental glass/polymer composite in the area of use of the filling, i.e. at temperatures between 30.degree. C. and 70.degree. C., is matched to that of the tooth material in order to ensure that the filling has adequate thermal shock resistance. In particular the change between cold and hot foods entails the risk here of the filling loosening due to such thermal shock stressing and of a gap thus forming between the filling and the tooth, representing a preferential point of attack for secondary caries. It is usual for the glass to have the lowest possible expansion coefficient, since this allows compensation for the relatively high thermal expansion of the synthetic resin binder.
The X-ray opacity of dental glasses or materials is, in accordance with DIN ISO 4049, quoted relative to the X-ray absorption of aluminium as the aluminium equivalent thickness (AlET). The AlET is the thickness of an aluminium sample which produces the same absorption as a 2 mm thick sample of the material to be tested. An ALET of 4 mm thus means that a glass plate with a thickness of 2 mm produces the same X-ray attenuation as an aluminium plate with a thickness of 4 mm. X-ray-opaque dental glasses are required to have an AlET of at least 4 mm. This ensures sufficiently good differentiation capacity between filling and tooth material on X-ray pictures when used as dental filling. Gaps which have occurred and caries can be recognised well.
Furthermore, good chemical resistance of the glass powder to water, acids and caustic lyes must contribute to a long life of the dental filling. Owing to possible toxic side effects, the use of barium constituents in the glass should be avoided, although these constituents produce good X-ray opacity. The use of lead-containing constituents is likewise undesired from toxic points of view.
DE 32 48 357 A1 describes a pulverulent dental material based on calcium aluminium fluorosilicate glasses (a) and metals (b) usual for dental purposes and further components, which is characterized in that it contains at least some of (a) as a sintered mixture with (b). The powders of (a) used consist of (% by weight, calculated as oxides) SiO.sub.2 20-60, Al.sub.2 O.sub.3 10-50, CaO 1-40, F 1-40, Na.sub.2 O 0-10, P.sub.2 O.sub.5 0-10 and a total of 0-20% by weight, calculated as oxides, of B, Bi, Zn, Mg, Sn, Ti, Zr, La or other trivalent lanthanide oxides, K, W and Ge.
U.S. Pat. No. 5,215,459 relates to the use of glass ionomer cements for controlled tissue regeneration. The composition ranges quoted for the glass powder correspond to those given in DE 32 48 357 A1 with in addition SrO as an optional constituent in an amount of 0-40% by weight, where CaO and/or SrO are at least 1% by weight. In order to make the glass X-ray-visible, from 10 to 20% by weight of La.sub.2 O.sub.3 can be added.
The glasses described in the abovementioned specifications have a relatively low total content of B.sub.2 O.sub.3, ZnO, ZrO.sub.2 and La.sub.2 O.sub.3 (.ltoreq.20% by weight).
U.S. Pat, No. 4,775,592 describes a fluoroaluminosilicate glass for use as dental glass ionomer cement whose surface has been post-treated with a metal fluoride or a fluoro complex salt. The complex post-treatment here serves to achieve the requisite processing properties and a high compressive strength of the cement. The composition of the fluoroaluminosilicate glass powder can be in a broad composition range. It is prepared by melting the components (in % by weight) SiO.sub.2 25-50, Al.sub.2 O.sub.3 15-40, F 10-40 and phosphate 0-20. F here can be introduced as the fluoride of Zn, Al, Y, La, Zr, alkali metals and alkaline earth metals, and phosphate as the phosphate of alkali metals, alkaline earth metals, Zn, Al, Y, La and Zr. Oxides of Y, La, Zn, Ti, Zr and alkaline earth metals can also be introduced into the glass. JP 61-215234 A claims a glass composition for use as glass ionomer cement, suitable as dental cement. A broad composition range consisting of a multiplicity of possible components is claimed. Nevertheless, the specification only allows the production of glasses having a relatively low refractive index in an additionally narrow range of from 1.46 to 1.60. Glasses having refractive indices in the range &gt;1.60 which is favourable for novel dental materials are not described. A Ba content of up to 35% of weight is possible. The Ba content of 20.31 and 3.92% by weight given in two examples is problematic for toxicological reasons and does not meet the requirements of modern dental glasses.
A striking feature in this specification and in U.S. Pat. No. 4,775,592 is the high claimed F content of from 10 to 40% by weight and the optional content of B.sup.3+ and P.sup.5+ of from 0 to 8% by weight each (JP 61-215234 A) or phosphate of from 0 to 20% by weight (U.S. Pat. No. 4,775,592). The production of a non-opacified dental glass having a high F content without the requisite presence of B.sup.3+ and/or P.sup.5+ is difficult.
U.S. Pat. No. 3,971,754 describes the production of a dental filling material using a barium-, zinc- and zirconium-free glass which, in order to produce X-ray opacity, contains oxides and carbonates of lanthanum, hafnium, strontium or tantalum in the range from 5 to 60% by weight.
JP 6-39031 A describes zinc-free, X-ray-opaque implant materials based on calcium apatite and strontium apatite glass ceramics. In the compositions described, the X-ray-absorbent component employed in virtually all cases is exclusively SrO; only in 2 examples is up to 5% by weight of ZrO.sub.2 used. Fluxing agents, such as Na.sub.2 O or B.sub.2 O.sub.3, are present in at most very small amounts (maximum 0.5% by weight).
JP 5-331017 A describes zinc- and zirconium-free glass powders for dental cements whose X-ray-absorbing action is based on the use of SrO and La.sub.2 O.sub.3 (up to 20% by weight).
DE 3788816 T2 describes a process for the preparation of radio-opaque, crosslinked poly(carboxylic acid) dental cement containing a fluorine-containing, zinc- and zirconium-free glass powder. The requisite X-ray absorption is produced by addition of from 5 to 35% by weight of SrO.
U.S. Pat No. 4,215,033 claims a dental resin composite consisting of a resin and a non-toxic, alkali metal- and fluoride-free filler, where the filler consists of a two-phase boroaluminosilicate glass and one phase is partly removed again. The glass can contain additions of SrO, CaO and ZnO or SrO/ZrO.sub.2.
DE 44 43 173 C2 likewise claims a barium-free dental glass with a high silicon content (from 50 to 75% by weight of SiO.sub.2) and good X-ray absorption.
Furthermore, DE 43 23 143 C1 discloses a barium-, zinc- and zirconium-free dental glass having high X-ray absorption and a refractive index n.sub.d.ltoreq.1.56 which has a composition, in % by weight, based on oxide, of SiO.sub.2 45-65, B.sub.2 O.sub.3 5-20, Al.sub.2 O.sub.3 5-20, CaO 0-10, SrO 15-35 and F.sub.2 --O 0-2. The good X-ray opacity is achieved here through a relatively high content of SrO.
The object of the invention is to find a barium-free, X-ray-opaque dental glass for use in dental glass/polymer composites and to provide a dental glass/polymer composite comprising such a dental glass. The dental glass and the dental glass/polymer composite should be inexpensive, but nevertheless high-quality and tolerated by the body, and should be suitable for passive and active tooth protection and should have excellent properties regarding processing, setting behaviour and strength.
The refractive index n.sub.d of the dental glass should be matched to the available dental polymers, particularly to those having a refractive index n.sub.d &gt;1.60, and so meet the aesthetic demands for a natural appearance made of a dental glass/polymer composite.